Usually when you flash paper, you give it just enough pre-exposure to non-image light so that the paper has been brought just to the threshold at which any further exposure will produce tone. The effect is to lower the contrast of the paper. It is therefore a technique of much more limited use with VC papers than with graded papers. With graded papers it could be used to get intermediate grades. And some people also used it so that they could use higher grades - which sometimes gave higher density blacks than lower grades (Richard Henry showed this effect with his meticulous tests of flashing with Ilfobrom).

With VC papers, there isn't much point in flashing an entire sheet. However it can still be a useful technique for bringing in stubborn highlights if applied locally (and there are various methods of doing that). When done in this way the effect is essentially the same as burning in with a very low contrast filter.

Actually I'm working on an article to show completely (including data) what flashing does (using MGIV), the differences between pre/post, and the effects of flashing at different grades. If there is interest on APUG I will post the article.

There are some threads here on flashing in the enlarging forum.